This invention provides an apparatus for reducing or eliminating pressure and flow rate surges in polymer extrusion machines. It also relates to a method of extruding with a substantially constant polymer pressure. In particular, this invention provides an extrusion apparatus and method utilizing a surge suppressor incorporated into the extruder screw.
1. Field of the Invention
Surges within polymer extruders are recognized as a major problem faced by the extrusion industry. Surges are output variations from an extruder screw corresponding to variations in polymer pressure and changes in polymer flow rate. Accordingly, surges are nearly synonymous in the extrusion industry with pressure and flow variations. Put simply, surges are like waves wherein maximum output and pressure occur at the top of the wave and minimum output and pressure occur at the bottom of the wave. When a wave-like surge arrives at the discharge end of the extrusion screw, there will be a corresponding surge in discharge pressure and flow rate. Accordingly, an instantaneous pressure or flow rate surge will produce an instantaneous surge at the extrusion die.
Pressure and flow variations at the extrusion die are know to result in dimensional variations in the extruded product. Such dimensional variations create severe problems, especially when it is desired or necessary to extrude tube, rod or other shapes having tight tolerances. Dimensional variations may result in the extrusion of large quantities of expensive materials into useless products. Moreover, pressure and flow changes at the extrusion die cause dimensional variations along the length of an extrusion. Inspection of one portion of the extruded product may result in different results from other portions, reducing predictability. Dimensional and other variations resulting from polymer surging ultimately results in material waste, product rejection, and other inefficiencies.
Various attempts have been made to efficiently and effectively control variations in extruder output. For example, valve control of extruder output was considered in Patterson et al, The Dynamic Behaviour of Extruders, SPE ANTEC, pp. 483-487 (1978). Also, control of melt temperature and pressure by continuously varying screw speed coupled with infrequent variations in die resistance was considered in Parnaby et al, Development of Computer Control Strategies for Plastic Extruders, Polym. Eng. Sci., Volume 15, No. 8, pp. 594-605 (1975).
Pressure controllers have been used in conjunction with gear pumps to compensate for extrusion pressure fluctuations. Gear pumps are sometimes positioned between an extruder barrel and an extruder discharge. Such pumps generally have precise outputs as well as a fixed resistance at a particular speed. However, increased extruder output has been known to generate very high pressure between the extruder and gear pump, sometimes causing the pump gears to seize. By use of a pressure controller, extruder screw speed may be reduced in an attempt to compensate for pressure increases and raised to compensate for pressure reductions. Examples of such pressure controllers include those provided by Dynisco, Sharon, Mass. (Model No. 660) and Barber Coleman, Loves Park, Ill. (Model No. 940).
Pressure controllers frequently utilize a pressure sensor positioned near the gear pump. Controllers are generally adjusted to ignore short-term pressure fluctuations and to correct extruder screw speed in response to long-term pressure trends.
Pressure controllers used in conjunction with gear pumps sometime cause unstable pressures when they attempt to correct for small or sudden pressure fluctuation. There is inherently some delay between the sensing of a pressure fluctuation and the actual screw-speed change initiated by the controller due to instruction delay and system inertia (i.e., inherent inertia of the drive motor, drive belts, transmission and other system components). Delay causes the controller instruction to be out of phase with the pressure fluctuation and therefore makes the pressure fluctuation worse.
Pressure controller instability is aggravated by the chaotic and unpredictable nature of extrusion pressure fluctuations. Small pressure increases or decreases often do not indicate continuing pressure increases or decreases. Pressure fluctuations are often quite short in duration.
Lee, in U.S. Pat. No. 4,118,163, also recognized difficulties in controlling the uniformity of pumping zone pressure and attempted to minimize pressure and flow surges at the discharge end of an extruder. Lee provided a complicated screw extruder apparatus having two separate pumping zones, a first zone for feeding plastic from a hopper and a second zone for pumping plastic back toward the first zone and out a lateral exit orifice. The first and second zones were connected by a central bore formed in the second zone of the screw which communicated with the first zone through radial passageways. The Lee extruder was an expensive device requiring a highly specialized extruder screw.
These attempts failed to provide a practical and effective apparatus or method for reducing polymer surging. Accordingly, there is a great and thus far unsatisfied demand for a practical apparatus and method for reducing or eliminating surging of polymer in extrusion processes.